Chapter 3: Fire and Metals

Prologue: the Pathetic Potter

Once upon a time there was a potter whose dreams ran away with his profits. Instead of turning out the usual jars for storing grain and oil, with a steady market and sure though modest marriages for his numerous daughters, the potter became bored, and ambitious. He would produce something different, something fine-looking, ornamented beyond the dreams of the villagers.

Now any potter could produce ornamented pots, using ochre powder rubbed on to the pot in patterns that had been used in the village as long as potters had produced pots there: at least ten generations, by all accounts. The ochre paste could be fired to remain a bright red, or to change to a deep black. But what if a potter could produce other colors?

Our potter began to experiment. Week after week, he spent hours on his project. He would trade for pretty minerals, grind them up into pastes, and spend hours painting them on pots in the most intricate and sacred motifs, or he would arrange beautiful crystals into the surface of the pot, set into beautiful patterns like the mosaics in the temple in town. Perhaps, thought the potter, he could send such fine work to the shrine of the goddess, gaining great favor with her and ensuring that he would not be a potter in the afterlife, slaving away over a hot kiln and trying to make a living in a village full of poor and surly peasants.

Week after week, the potter used valuable space in his kiln to firing his experiments, and week afer week he would throw away the useless results. His crystals would turn into ungly great blobs, and the beautiful green and blue pastes would turn into spotty red and yellow smears. The potter became despondent, and even the quality and quantity of his normal work dropped off. At harvest time people had to go to the next village to find enough jars for the new crop. The potter, worn out by his failures and the nagging of his wife and daughters, gave up his dream and lived unhappily ever after. The least repulsive of his sons-in-law learned slowly, but in time came to make acceptable jars, good enough to keep at least one of his daughters from starving. But the potter never mentioned his experiments again, and from that day to this the potters of the village have stayed strictly with the patterns and pigments they learned from their predecessors.

The Discovery of Copper

Copper is easily worked, and it occasionally occurs as a pure metal, sometimes in quite large masses. However, it is so soft that it is comparatively useless for most tools and weapons in its native state. By the time we see the first copper artefacts, stone technology was so advanced that stone tools were better than copper for most applications. But copper is a beautiful metal: even a raw copper nugget is a shiny rich tawny color if it has been cleaned and polished (by rolling round in a stream, or by a collector). As a result, copper was often used in early times for ornaments and display objects (luxury items) rather than for any practical utility. At Çatal Höyük, small copper plates were used as decorations. Beads, rings, and bracelets of native copper were made in ancient Egypt, earrings in Virginia, head-dresses in the Mound cultures of Ohio, and rings and bracelets in British Columbia. Copper can be worked into some small useful tools and objects that are awkward to fashion out of stone (awls and hooks), but even then, fresh bone is fairly easy to work, easy to get, and hardens with age, while copper is always rather soft, which makes it useful for electricians!

A major prehistoric industry exploited copper round the Great Lakes of North America. Native copper occurs here in unprecedented quantity, sometimes in nuggets weighing more than 100 kg (220 pounds). More than 10,000 separate excavations by early native Americans have been identified, some of them dating back to 3000 BC, and thousands of tonnes of copper were probably mined. The copper was mined and then worked with stone hammers, largely for articles of trade and ornament: in this region it did not provide a pathway to more advanced metalwork. The lack of technology (no high-fired ceramics, for example) and the lack of suitable alloying metals meant that there was no development of smelting or metallurgy in this region.

Çayönü Tepesi

Because copper is comparatively soft, it can be worked by cold hammering with stone tools. Some early copper smith must have discovered that "cold working" can more than double the hardness of copper, to the point where it equals pure iron, though then the copper is brittle and cracks very easily. However, some gentle heating will anneal the copper, which can then be cold-worked again to give a fairly heavy tool with a comparatively hard surface that can, if desired, be shaped to form a sharp edge. From the hands of skilled smiths, small copper tools may have been better than commonly supposed.

The earliest known metal objects date from around 7000 BC, and come from several sites in the upland plateau of Anatolia, now in modern Turkey. The best known objects are copper hooks and awls from Çayönü Tepesi, in eastern Turkey. The copper at Çayönü Tepesi came from an ore deposit at Ergani Maden, only 20 km away, which is still worked for copper today. The Çayönü Tepesi site contains some of the very earliest attempts at working native copper with fire (pyrotechnology), instead of cold-working it. The hooks and awls had been heated and hammered into shape, but the smiths clearly did not understand how annealing and cold-hammering affected copper, because they were left in the softened state, without the final hammering. Other copper objects are known from Asikli Höyük, sheets of native copper hammered flat.

The River Tigris is navigable by raft or by hide-covered boat all the way down river from eastern Anatolia into the plains of Mesopotamia, and it is perfectly reasonable to suggest that some of the early copper objects from lowland sites dated between 5000 and 7000 BC in Mesopotamia came from the Anatolian highlands to the north.

Çatal Höyük

Most likely the original success of this city was based on its obsidian industry, as we have seen. However, Çatal Höyük is important in terms of the history of metal because the site has yielded copper and lead artefacts dated about 6000 BC. The copper had apparently been hammered into little plates and used to decorate the edge of a piece of cloth, and the lead had been made into 13 beads strung on a necklace. At Çatal Höyük, therefore, agriculture, ceramics, stone-working (for obsidian), and metal-working supported a truly urban settlement, and a society that looked outward to trade extensively with its neighbors.

Çatal Höyük is a good example of the transitional stage between the Neolithic and the Bronze Ages: the Chalcolithic. Metal-working has begun, but copper, not bronze, is the dominant metal, and most tools (and probably all weapons) are still made of stone. However, Çatal Höyük declined with the transition from the Neolithic into the Bronze Age, and it is tempting to see the problem as geologically based. Çatal Höyük prospered by its good fortune in access to obsidian; but it was not suited to be a center for extensive metal-working. Why not?

Pyrotechnology and Fuel

The control and application of fire is, of course, one of the oldest human technologies. The application of fire on a large scale (pyrotechology) marked the first truly industrial activities, and we can see the first economic, historical, and environmental repercussions of fuel use. The first fuel for pyrotechnology was wood, rather than the geological fossil fuels, but I discuss it here because its use led to all kinds of other geological effects on human history.

One regional feature of the ancient civilizations of the Mediterranean and Middle East was their furnace technology: it was apparently not duplicated elsewhere in the world at this time. Certain materials were used individually or together in furnaces, and the various combinations allowed experiments (sometimes accidental) that led to the full development of pyrotechnology.

The first use of fire for "industrial" purposes may have been fire-treatment of flint, chert, and obsidian. If nodules are heated on an open campfire to 100­400° C and allowed to cool, they are a little easier to work, giving less wastage, though the tools produced would be slightly softer and less hard-wearing. (One can imagine ads for tools made from "natural" versus processed flint!). It would depend on the eventual use of the tool whether heat-treatment was a good idea.

However, flint treatment would not have been a large industry. The first pyrotechnology that made a significant impact on society was probably lime-burning. In this process, limestone (or sometimes oyster shells if there is no limestone nearby) is smashed into small pieces, and fired in a kiln (made of stone blocks), with layers of wood between the layers of crushed stone. At temperatures around 800-900° C, the limestone is "calcined", that is, it breaks down:

CaCO3 = CaO + CO2

Once the fire has died down and the calcined lime dragged out, the pile of powder is mixed with water to produce calcium hydroxide or "slaked lime":

CaO + H2O = Ca(OH)2

Over time, calcium hydroxide is unstable, and breaks down by reacting with carbon dioxide in the air, expelling water vapor invisibly as it does so:

Ca(OH)2 + CO2 = CaCO3 + H2O

This last reaction allowed people to make plaster. Damp calcium hydroxide would be mixed with sand as a cheap filler to make mortar that could be used to bind, fill, and eventually strengthen a stone wall, or it could be mixed with limestone chips (or other stones) to make a terrazzo flooring.

The lime-burning and plaster industry predates pottery-making. Around 6500 BC, the people of Çayönü Tepesi were laying terrazzo floors in their houses: gray and red limestone pieces had been laid in a matrix of lime mortar, and carefully polished. Calculations suggest that it would have taken at least half a ton of plaster per house for the flooring at Çayönü Tepesi, which involves burning a lot of lime and consuming a lot of wood fuel.

Next, pyrotechnology was extended to pottery. In the reactions inside the kiln, the aluminosilicate minerals that are "clays" react to form more refractory compounds that are much harder and stronger, heat-resistant, and, with a glaze, air-tight. Potters would quickly have found that the quality of pottery increases with firing at higher temperatures, and would have built improved kilns to achieve those higher temperatures. Potters would also have found that charcoal gave more heat than dry wood, though taking up a much smaller volume in the kiln. This allowed better pottery produced in larger batches. In all, pyrotechnology had now reached a stage where the almost universal use of pottery and pottery kilns, plaster and terrazzo, must have made a significant impact on local woods and forests.

I suspect that the decline of Çatal Höyük is probably related to deforestation. As we saw in Chapter 1, trees grew very slowly around Çatal Höyük. Perhaps the area simply didn't have the fuel to support metal-working. Çatal Höyük may have been the first example, but was certainly not the last, of a society in which deforestation led to economic decline or disaster.

Copper Smelting: the Case of the Perspicacious Potter

In Anatolia there was not the abundant supply of native copper that the North Americans found round the Great Lakes. After the first scraps of native copper had been worked out, copper had to be extracted from copper-bearing minerals by smelting. Smelting is not to be confused with melting: smelting is the process by which fire is used to extract metal from ores such as oxides, carbonates, or sulfides.

We not know how copper smelting was discovered, and it cannot have been easy to recognize that it had happened. For example, the vivid green copper ore malachite must be heated in an oxygen-starved fire before molten copper beads are formed. The usual story is that someone discovered accidentally that certain stones produced molten metal if they were used in building a camp fire. However, most campfires are small, open, and short-lived. They burn at most at 600° or 700° C, and hardly ever produce the right oxygen-starved conditions for smeltingso this cannot be the true story.

There are two plausible pathways for the discovery of smelting, both of them involving pottery kilns. For most of the classic Stone Age, sites are without pottery. The discovery of pottery for beakers and bowls was made in Western Asia perhaps around 9000 BC, and it is probably not a coincidence that pottery-making was followed relatively quickly by the use of metals. Firing clay into useful pottery demands the careful production of high temperatures inside a kiln. Pottery kilns can reach much higher temperatures than open fires, and they operate somewhere near the boundary between sufficient oxygen and oxygen starvation. Loaded properly, kilns can maintain temperatures above 1000° C for hours, in an oxygen-starved atmosphere. Firing to about 450° C makes pottery hard and waterproof: firing to 1400° C makes the pot shiny and even harder, and this temperature can smelt metal out of an ore.

Galena, the sulfide ore of lead, is a rare but flashy mineral. It is bright and shiny when its surfaces are freshly exposed, and anyone breaking open a rock containing galena would notice it immediately. Galena is brittle, however, so that hammering simply breaks it into even tinier fragments, in contrast to copper, which is soft enough to be molded by hammering. Galena is not only bright, but metallic-looking, and some early experimental genius may have tried to heat and hammer galena, thinking it was a metal. (However, anyone who tries to heat galena on an open flame quickly finds that it gives off unpleasant fumes. I tried it in my bedroom with a candle when I was a boy, and the crystals crackled and spat white sulfur dioxide fumes in my face.) In chemical terms, the reaction is

PbS + 3O = PbO + SO2

A potter would not have looked at galena as a material for metal-working. But he might have tried embedding crystals of galena into the surface of a pot, for ornamental effect. Fired in a pottery kiln, under conditions of limited oxygen, galena does not "burn" as in the reaction above. In modern chemical terms, it is smelted to produce pure beads of lead, because there is not enough oxygen to oxidise the lead to the uninteresting lead oxide. The reaction here is

PbS + 2O = Pb + SO2

The potter would have been disappointed by the dull grey beads of lead on his pot. Far less decorative than the original shiny crystals, the metallic lead would not have any further interest for pottery. But a perspicacious potter, or his colleague the smith next door, might have investigated the new substance, and could quickly have discovered that this was a metal, that it was not copper but was even denser and softer than copper, and it could be worked easily. In this scenario, then, smelting was discovered by attempts to use galena as a decorative mineral.

More fundamentally, the discovery implied for those that could see it that there were minerals that were not metals themselves, but could be made into metals by firing. The potter had not simply discovered a metal he was familiar with, but a metal completely new to him, that did not occur naturally as a metal. This discovery would surely have led someone to experiment with different minerals, particularly with colorful ones, and sooner or later someone found that the familiar metal copper did not just occur as native copper, but could be smelted from other minerals too. Furthermore, copper-yielding minerals were easy to identify, more common than native copper, and could be smelted in kilns that were already being used to make high-quality grades of pottery.

Perhaps a perspicacious potter discovered copper smelting directly, rather than systematically following up the discovery of lead. He may have been experimenting with copper-bearing minerals for ornamental pigments and glazes: once again we are dealing with art, rather than simple utilitarian pottery. The copper ores malachite and azurite are bright green and bright blue respectively. An early potter experimenting with powdered malachite and azurite as pigments, firing them to different temperatures, might have accidentally produced small droplets of molten copper, even if the kilns did not reach much over 1000° C.

Here is the chemistry that goes on, invisibly, inside the kiln. Malachite and azurite are basically copper carbonate minerals, Cu2(CO3)(OH)2 and Cu3(CO3)2(OH)2 respectively. As the kiln heats up, the carbonates begin to break down to release water vapor and carbon dioxide, and in doing so convert to a copper oxide,

Cu2(CO3)(OH)2 = 2CuO + CO2 + H2O

Cu3(CO3)2(OH)2 = 3CuO + 2CO2 + H2O

This would have been interesting to the potter even if this is as far as the reactions went. Copper oxide is red to black, rather than the blue and green of the original powders, and the completeness of the change would vary, depending on the temperature of the kiln and the length of the firing. The potter would have continued to experiment with malachite and azurite, each time producing a small amount of copper oxide, CuO.

If copper oxide is fired to high temperature in an oxygen-starved kiln, large quantities of hot carbon monoxide are produced in the kiln as charcoal burns to CO rather than CO2. Carbon monoxide attacks oxides, ripping off an oxygen atom to form CO2. At about 1100° C, copper oxide forms copper metal:

CuO + CO = Cu + CO2

Once malachite and azurite were identified as significant sources of the well-known (and valuable) metal copper, potters may have deliberately built kilns dedicated to smelting copper ores, and it would not be long before the basic design of the kiln was modified to make an effective smelting furnace, operated by a new kind of craftsman who was a specialized smelter. The smelter would pack charcoal in intimate contact with crushed ore, to ensure more complete chemical reactions in more reliably oxygen-starved conditions immediately round the ore. The working temperature was raised by blowing air into the furnace through specially designed pipes called tuyères, with or without bellows. Once the copper metal had melted, it would flow downwards to be collected in a ceramic container or crucible. Such furnaces could reliably reach temperatures that would smelt high-quality copper ores.

What does the archaeological record tell us? There is no pottery at Çayönü Tepesi at 6500­7000 BC, though there was lime-burning and terrazzo flooring. At Çatal Höyük, lead beads are associated with a culture that was working native copper but had not yet discovered copper smelting. This suggests that the people of Çatal Höyük or their suppliers had worked out the principle of smelting, but temperatures were not yet high enough to smelt copper. There is ornamented fired pottery at Çatal Höyük around 6000 BC, suggesting strongly that kilns were already in use.

As specialized furnaces were built, smelters quickly learned to handle molten metals. (Note that the only practical early containers for molten metal would have been clay molds or ceramics). Early pyrotechnology quickly came to include metal casting. The first known cast copper object is a mace head from Can Hasan in southern Anatolia, from about 5000 BC. By this time there would have been specialized and skilled smelters using specially designed furnaces, and smiths would had become separate craftsmen.

The Spread of Mining and Smelting

Metal working can be done on a village scale, once the craft is passed along. But in the right conditions, larger-scale operations became possible, in mining, smelting, and metal-working. Such changes require a supply of the right ores, skilled craftsmen, and fuel for the smelting, casting, and smithing. They also require demand for the finished goods, and the transport and trading infrastructure that delivers goods from maker to user. But those conditions were present in many areas in and around the apparent origin of metal-working in Anatolia. We can begin to use the word industry to describe some of the operations that archaeologists have discovered.

Copper-working had spread to the Balkans and the Aegean by 4500 BC. Copper tools, weapons, and ornaments were plentiful in the Balkans at this time. Many copper (and gold) objects have been excavated from graves at Varna, on the Black Sea coast of Bulgaria, dated at 4300 BC. Copper working had probably been an industry in this region for centuries, and smelting malachite had been routine for some time.

Somewhat later, but before 3000 BC, large copper mines were opened on the eastern side of the Jordan valley, at Fenan, a site where several wadis drain from the Jordanian plateau into the rift valley. More than 100 galleries were driven up to 10 m into the valley sides, mainly to extract nodules of malachite. The miners' huts are still preserved on a high terrace above the valley floor.

Chalcolithic mining in the Balkans, in the Jordan Valley, and elsewhere in the ancient world may have worked out much of the more easily mined and smelted oxides and carbonates of copper. Later miners, smelters, and metallurgists had to face the problems of extracting copper from deeper levels and from more difficult ores such as copper sulfides. Possibly the beginning of the Bronze Age marks the first time at which people learned to smelt copper from sulfide ores, thus releasing new sources of copper to replace the simpler Chalcolithic supplies: native copper, and copper smelted from oxide and carbonate ores.

Copper sulfide ores have much more complex chemistry, and the copper smelted from them tends to contain a variety of impurities, some of which "improve" their properties in terms of tool, weapon, or even ornamental function. Technology evolved from Chalcolithic Age to Bronze Age as miners, smelters, and smiths learned to use these new and more complex materials, especially to form deliberate alloys.

The first smelters would have used the first, best surface ore. As smelting became more widely used, as demand for lead and copper increased, smelters would have had to use whatever ore they could, even it it was clearly not the best. If a batch of ore is not pure, smelting may not work well. The reactions inside the furnace might produce copper, but if there was too much iron ore or quartz in the ore, it would remain in the copper mass and make it useless for further working. The furnaces were not hot enough to melt these impurities so that they would separate from the copper. The ancient smelters solved the problem in practical terms, but the thermodynamics that explains it was not understood until the late 19th century.

At some point, a smelter discovered that adding some extra ingredient (a flux) would help the smelting process. The flux interferes with the smelting reactions in a beneficial way, forcing the impurities to react to form more complex compounds that would melt inside the furnace. Once they melted, the new compounds would separate from the denser molten copper and float upward to form a layer of waste (slag) on top of it. The smelter would have set clay plugs into the walls of the furnace, and by breaking them open could tap off the slag while it was still molten, running it into a waste clay-lined pit at the side of the furnace.

Fluxes would have been discovered fairly quickly and easily. By great good fortune, it turns out that one of the best fluxes (even today) is fayalite, a mineral which has the chemical formula Fe2SiO4. Fayalite is extremely rare in the rocks of the Earth's crust, but it turns out that it is chemically just a 1:1 combination of quartz (SiO2) and our old friend hematite (or ochre), Fe2O3. Given that quartz and iron oxide are very common minerals, smelters would have found that certain ores came with impurities that made the copper ores "self-fluxing". Once someone had noticed that, the smelter would quickly learn to add ochre to ores with silica impurities in them, and to add crushed quartz or sand to iron-rich smelter loads, to achieve just the right mixture to remove both of them from the copper as slag. At 1120° C, the right mixture of fluxes would form a fayalite liquid much lighter than molten copper, and the problem would be solved.

Copper smelting was invented locally in several different places. It was probably discovered independently in China before 2800 BC, in Central America perhaps around 600 AD, and in West Africa about the 9th or 10th century AD. By about 750 AD the technology reached Western Mexico, where once again copper was used for ornamental, artistic, and religious purposes, as pendants, beads, and bells. Copper working reached northern Peru about 800 AD. At Batan Grande, in the Andes, ores and slags show that copper was being smelted in about 100 bowl furnaces, all arranged so they had wind-assisted firing.

The Magic Potters of Dolni Vestonice

I cannot resist telling this story! It turns out that the oldest pottery we know was made for art or magic, not for utilitarian purposes. The Gravettian people of Eastern Europe were making pottery figurines over 20,000 years ago, in a particularly interesting way. Literally thousands of figurines, most of them models of animals, were discovered at Dolni Vestonice, in modern-day Slovakia. But almost all were shattered into fragments. Experiments using the same loess soil that the Gravettian potters had used for their "clay" showed that it was perfectly possible to fire good figurines from the loess, in kilns reconstructed from those on the site. However, if the figurine was made with wet clay on the inside, wrapped with normal damp clay, then fired, it would almost always explode in the kiln, producing a most satisfactory bang and a handful of fragmentary objects just like those found at the Dolni Vestonice site.

The Gravettian potters, then, deliberately made their figurines to explode! This has been described as the first "performance art," but for the Gravettians it probably had great magical symbolism, perhaps getting rid of demon figures. Given that the sacrifice of the figurines could be done only in the kilns, one can imagine the pilgrims lining up to buy their offerings at the gift shop, then offering them ceremonially and reverently to the priest-potters who had made them (at a handsome profit)! If you think this comment is cynical, watch the Catholic pilgrims at Lourdes lining up to buy candles and plastic Jesus and Mary figurines!

The first pottery, made out of loess
Is in pieces, or fragments, or worse,
They mixed up wet clay
Fired it up right away
And blew it to bits with a curse.